What is Defection?

Legal Definition
In politics, a defector is a person who gives up allegiance to one state in exchange for allegiance to another, in a way which is considered illegitimate by the first state. More broadly, it involves abandoning a person, cause or doctrine to which one is bound by some tie, as of allegiance or duty.

This term is also applied, often pejoratively, to anyone who switches loyalty to another religion, sports team, political party, or other rival faction. In that sense, the defector is often considered a traitor by their original side.
-- Wikipedia